The present invention relates to an apparatus and method for printing a dot matrix of characters on a record member and, more particularly, to an apparatus and method for substantially increasing the speed of such printing without reduction in the quality of the printed characters.
In a printer for printing a dot matrix on a record member, print or output ends of wires are held in a printhead in a fixed array. The printhead is fixed to a carriage which typically moves within a limited range along a track to successive printing stations. At each printing station, a predetermined number of wires are actuated to strike the record member through an inking ribbon to form a portion of a dot matrix of a character on the member. To actuate the print wires, electrical signals are generated to energize a determined number of electromagnets which control hammers or "clappers" which propel the wires to move them towards the record member. Both the wires and their electromagnets are embodied in the printhead, known as a matrix printhead, which typically has a single column of, for example, seven or nine wires, facing the record member.
It is, of course, desirable to design a wire printing system which can form characters as quickly as possible. In the present state of the art, the frequency of impact of a wire on a record member is about 1,000 impacts per second, which produces about 200 characters per second. This operating speed is limited by the minimum interval allowable between successive impacts of each wire, with such minimum interval being limited for a number of reasons. First, a sufficiently efficient electromagnetic material is not presently available to move the clapper fast enough to propel its associated wire to cause an impact rate significantly higher than about 1,000 cycles per second. Second, even if such material were presently available, the dynamics of the wire and clapper would impose upon the design of the printhead severe constraints such as allowable settling time, material strengths and wear characteristics of the wire and clapper. Alternately, to generate a strong magnetic field without an efficient magnetic material would impose severe restrictions on the design of the drive circuit for the clapper, power consumption, reliability and the economics of the printing system.
One system for increasing the rate of character formation includes operating the printhead wires at the present limit of about 1,000 impacts per second, but which provides two separate printheads, each printing half a line, thereby doubling the rate of character formation. In such a system, the two printheads, each having a single column of wires and movable together, are operated simultaneously so that while one printhead is printing the first half of a line, the other printhead is printing the second half of the line. This two printhead system has a number of disadvantages as will now be described.
In many instances, a dot matrix printer may not be printing a full line on a record member such as a sheet of paper. In this case, in the two printhead system the left printhead, for example, will be performing more work than the right printhead, the percentage of more work depending on the percentage of a full line being printed. If only half a line is to be printed, the right printhead will move along the record member, but will not perform any printing. Consequently, it is not unlikely that the left printhead will wear out much faster than the right printhead, thereby requiring significant time and cost in providing a new printhead and readjusting the machine for this new head. Also the use of two separate printheads requires additional manufacturing cost for hardware and additional physical space on the dot matrix printer, thereby adding additional design constraints. Perhaps even more importantly, the throughput or character formation of this machine when printing, for example, half a line, is still only that of the single head machine described above.
Furthermore, with a two printhead printer, line buffering is necessary in order to print a full line. That is, data incoming to the dot matrix printer and representing a full line of characters must be stored in two separate half-line buffers before the printer can be activated to print such a line. This is because the data for the first part of the left half of the line must be sent to the left printhead when data for the first part of the right half of the line is sent to the right printhead since the two printheads must be activated simultaneously to print both halves of the line simultaneously. Also, the dot matrix printer having the two separate printheads is relatively more difficult to control when it is responding to character at a time key-board data entry. To key in a whole line of information, the left printhead moves right to record the left half of the line, then the two printheads are returned to their original left position, printhead selection is then switched to the right printhead, and then keyed in data will be printed on the right half of the line with the right printhead.
In addition, in a dot matrix printer, the gap between the ends of the printhead wires and the record member must be accurately adjusted to achieve suitable print density. That is, this gap must be adjusted so that the wires will strike the record member through the inking ribbon with appropriate force to place a pronounced dark dot on the member. When two separate printheads are used, their gaps should be precisely the same; otherwise, for example, the left printhead might print characters which are darker than those printed by the right printhead, resulting in an overall appearance which is not pleasing. Since these gaps may be, for example, as small as 0.014 inches with tolerances of only .+-. 0.001 inches, it is oftentimes difficult to match accurately the gap distances of both printheads.
Yet another disadvantage of the dot matrix printer having two printheads is that the width of the record member on which it can print is limited if advantage is to be taken of the increased rate of character formation which such printer can provide. If a sufficiently wide paper is not used, then, for example, only the left printhead will be useful while the right printhead will not be energized. Thus, again the rate of character formation of the two printhead machine will be reduced, for example, to that of a single printhead machine described above.
Furthermore, as indicated above, the two separate printheads must be operated simultaneously to print respectively one half of a line. This means that the wires in both printheads will be energized simultaneously, thereby increasing the peak power requirements of the machine.